1. Field of the Invention
The present invention pertains to ventricular assist devices, and more particularly to artificial prosthetic conduits used for transporting blood in the circulatory system of a living organism.
2. Description of the Prior Art
More than two and one-half million Americans suffer from congestive heart failure. Most heart disease involves the left ventricle of the heart. This pumping chamber is generally known as the workhorse of the heart. A patient with a non-functioning right ventricle can survive quite successfully provided that their pulmonary blood flow resistance is low enough to allow circulation through the lungs and the rest of the body entirely as a result of the efforts of the left ventricle. However, collapse of the left ventricle is most often fatal.
Left-ventricular assist devices (LVAD) in particular are recognized as potentially very valuable for assisting patients who suffer from congestive heart failure. An LVAD is able to fully take over the function of the left ventricle, thus perfusing the body with oxygen-rich blood. The LVAD attaches to the patient's natural heart, and to a natural artery, and can be removed if the natural heart recovers. Some LVADs are surgically implanted into the patient's abdominal cavity, while others remain outside the body and are placed in fluid communication with the heart via elongated cannulas. Recently, a National Institutes of Health study estimated that as many as thirty-five thousand people could be candidates for use of a left-ventricular assist device.
At present, conventional ventricular assist devices are used for patients a) who are waiting for a heart transplant (a so-called, "bridge to transplant"), b) whose natural heart is of such poor condition that the patient cannot be removed from a heart-lung machine without providing some assistance to the patient's heart following otherwise successful open-heart surgery, and c) who suffer massive heart attacks that lead to circulatory collapse. The suitability of long-term utilization of conventional left-ventricular assist devices outside of the clinical environment remains under study.
Expansion and contraction of a variable-volume chamber typically effect blood flow in the LVAD. One-way valves associated with the inflow and outflow ports of the LVAD permit blood flow propelled by the natural left ventricle into the variable-volume chamber during expansion, and blood flow out of this chamber, usually to the ascending thoracic aorta. These one-way flow valves may be constructed as part of the LVAD itself, or may be disposed in separate blood-flow conduits attached thereto. A pair of artificial blood conduits respectively connect the inlet port of the variable-volume chamber (or the inlet end of a valved conduit) to the left ventricle and the outlet port of the variable-volume chamber (or the outlet end of a second valved conduit) to the major artery which is to receive the blood flow from the device.
As is well known, artificial blood conduits have become a valuable tool of modern medicine. One use of such artificial blood conduits is as a temporary or permanent prosthetic artery. Another use is in the connection of temporary blood pumps, such as ventricular assist devices described herein, between the left ventricle of the heart and a major artery.
The demands on artificial blood conduits in ventricular assist devices are great. The conduit must deal with the pulsatile blood flow created by the host's own heart, as well as with the flow, pressure, and pulsations created by the assist device. Moreover, there are differences in flow and pressure between the inflow and outflow conduits connected to the pumping device. For example, while the outflow conduit experiences regular pulses of high pressure, flow in the inflow conduit is dependent on the pumping strength and rhythrm of the natural left ventricle on top of which the periodic LVAD pressures are superimposed (i.e., expansion of the variable volume chamber tends to pull fluid from the inflow conduit). The inflow conduit thus sees irregular and typically low flows and pressures; indeed, negative pressure transients can occur in the inflow conduit.
Conventional artificial conduits for use in LVADs may be constructed of an elongate flexible woven polyethylene terephthalate fabric tube. In some cases, the conduits are sealed with a thin bio-compatible collagen coating on the inner lumen wall to render the fabric more leak resistant at the time of implantation, and also more compatible with the patient's blood. The collagen coating, typically bovine collagen, eventually is absorbed into the blood stream and is replaced with a natural coating of blood cells, serum protein, and other elements from the blood. In the absence of a sealant, the conduit may have to be pre-clotted by the surgeon just prior to implantation. The woven fabric tubes for implanted LVADs are invariably convoluted (crimped) to facilitate bending and extension during implantation to fit different anatomical configurations. That is, the pumping device must reside with the lower abdominal cavity and attach via the conduits to appropriate locations on the heart, none of which are precisely the same in each patient. The convoluted conduits accommodate this variability without kinking. A conventional artificial blood conduit is disclosed in U.S. Pat. No. 5,810,708, issued Sep. 22, 1998, to Woodard.
Some non-implantable ventricular assist-devices utilize cannula-like conduits that are relatively rigid, some being fonned of smooth, reinforced polyurethane. These types of conduits would not be suitable for use in implantable devices as they will not easily accommodate varying anatomical placements, and tend to kink if bent. In addition, smooth-walled woven fabric grafts are relatively stiff, and tend to kink when bent.
In spite of extended efforts in the industry, there remains room for improvement in the construction and function of conduits for ventricular assist devices.